1. Field of the Invention
The present invention relates to acoustic refrigeration apparatuses, and particularly to an acoustic refrigeration apparatus directed to high efficiency and simplification in the structure of the apparatus.
2. Description of the Background Art
An acoustic refrigeration apparatus that effects refrigeration utilizing acoustic waves is conventionally known (for example, refer to Japanese Patent Laying-Open No. 58-52948: U.S. Pat. No. 4,398,398).
Referring to FIG. 8 showing an acoustic refrigeration apparatus 200, there are provided a resonance tube 202 having one end 202A closed and the other end 202B open, a speaker 201 opposite open end 202B of resonance tube 202 for acoustic generation, and a regenerator 203 having a plurality of layers of flat plates arranged within resonance tube 202.
The frequency of the applied current to speaker 201 is set so that the acoustic wave resonates in resonance tube 202. Upon generation of an acoustic wave from speaker 203 towards closed end 202A of resonance tube 202, a pressure distribution P is generated as shown in FIG. 8, exhibiting alternate generation of an antinode of great pressure variation and a node of small pressure variation. Also, the antinode and node are generated as to the gas displacement, as indicated by the arrow W in FIG. 8.
As a result, difference in temperature occurs at respective ends of regenerator 203. The low-temperature end and the high-temperature end of regenerator 203 effects cooling of the object of interest and heat rejection outwards via respective heat exchangers (not shown).
The cycle acoustic refrigeration apparatus 200 undergoes can be defined by the Brayton cycle including the four steps of adiabatic compression, isobaric change, adiabatic expansion, and isobaric change of a parcel of gas.
In the Brayton cycle provided by conventional acoustic refrigeration apparatus 200, heat is absorbed and rejected according to the difference between the temperature when the parcel of gas is expanded and the temperature of regenerator 203 and between the temperature when the parcel of gas is compressed and the temperature of regenerator 203, respectively. Therefore, the heat transfer process is irreversible. Thus, there is the disadvantage that the thermal efficiency is lower than that by the Carnot cycle.
The applicant of the present application has proposed an acoustic refrigeration apparatus that allows a reversible heat transfer process to realize a gas cycle approximating the Carnot cycle which is an ideal gas cycle (Japanese Patent Laying-Open No. 10-325625).
The basic structure and principle of this acoustic refrigeration apparatus will be described hereinafter with reference to FIG. 9 to FIG. 14D.
Referring to FIG. 9, an annular tube 1 in which an acoustic wave travels forms a channel of a hollow annular closed loop. The path length of annular tube 1 is set to be an integral number of the wavelength of the acoustic wave. In the following, it is assumed that the path length corresponds to the axial line in annular tube 1. Speakers 2 and 3 serving as acoustic wave generation devices are provided apart from each other by a distance equal to an odd-number of the quarter wavelength of the acoustic wave, and attached to annular tube 1 to emit an acoustic wave into annular tube 1.
An acoustic wave generation control device 50 is attached to speakers 2 and 3 to provide control so that the phase of the acoustic waves emitted from speaker 2 is delayed by the odd-number of the quarter period of the acoustic wave behind that from speaker.
The operating principle of the acoustic refrigeration apparatus will be described with reference to FIG. 10 here.
The acoustic wave issued from respective speakers 2 and 3 branches into two directions upon entering annular tube 1 to travel to the opposite directions respectively. The two progressive waves emitted from speakers 2 and 3 and travelling within annular tube 1 are superimposed with each other.
From the relationship of the arranged distance between speakers 2 and 3 and the phase difference of the acoustic waves, acoustic waves 2L and 3L travelling leftwards in the drawing go in phase to be amplified. Acoustic waves 2R and 3R travelling rightwards in the drawing go in antiphase to cancel each other. As a result, only the acoustic waves traveling in one direction (leftwards) remains. The remaining acoustic wave further circulates annular tube 1 to be further superimposed and amplified with an acoustic wave traveling behind in phase, resulting in increase of the amplitude as in the case of resonance.
Referring to FIG. 11, a regenerator 40 having high heat transfer rate and low pressure loss is provided within annular tube 1 of the acoustic refrigeration apparatus. The refrigeration principle thereof will be described with reference to FIG. 12.
The phase of progressive acoustic wave travelling through regenerator 40 is varied by the positions thereof. Focusing on a parcel of gas located at a certain position, an expansion change occurs when the parcel is displaced from its equilibrium position in the direction of the acoustic wave traveling and a compression change occurs when the parcel is displaced from its equilibrium position in the opposite direction of the traveling acoustic wave. By heat absorption and heat rejection by means of regenerator 40 in the expansion change and the compression change, the heat will be sequentially conveyed in the opposite direction of the traveling acoustic wave. Since this heat transfer process is reversible, thermal efficiency becomes higher than that of the conventional acoustic refrigeration apparatus.
The gas cycle of the above acoustic refrigeration apparatus will be described hereinafter with reference to FIG. 13 to FIG. 14D.
The Carnot cycle is constituted by an isothermal change and an adiabatic change. As shown in FIG. 13, the T-S diagram of the cycle is indicated as a rectangular shape diagram of A.multidot.H.multidot.G.multidot.D. A.fwdarw.H represents an adiabatic expansion change (constant entropy). H.fwdarw.G represents an isothermal expansion change. G.fwdarw.D represents an adiabatic compression change. D.fwdarw.A represents an isothermal expansion change.
In the case where the acoustic wave passes through in one direction a regenerator having superior heat transfer rate with a gas, pressure change occurs simultaneous to the reciprocation of the parcel of gas, as shown in FIG. 12. The pressure increases most rapidly and a parcel of gas is compressed most intensive when the parcel of gas passes through the most distant point from the equilibrium point in the direction of the acoustic wave traveling. Since the heat transfer rate of the regenerator is superior, isothermal compression is effected. This isothermal compression change is represented by D.fwdarw.A in FIGS. 13 and 14A.
When the parcel of gas is going on the way to the opposite direction of the traveling acoustic wave, heat is rejected along the temperature gradient of the regenerator. The parcel of gas is cooled down at an approximate isovolumetric change. This change is represented by A.fwdarw.B in FIGS. 13 and 14B.
When the parcel of gas passes through the most distant point from the equilibrium point in the opposite direction of the traveling acoustic wave, the pressure decreases most rapidly and the parcel of gas is expanded most intensively. This corresponds to the isothermal expansion change in which heat is absorbed from the regenerator. This stroke is represented by B.fwdarw.C in FIGS. 13 and 14C.
Similarly when the parcel of gas is going on the way to the direction of the traveling acoustic wave, isovolumetric change is exhibited in which heat is absorbed along the temperature gradient of the regenerator. This change is represented by C.fwdarw.D in FIGS. 13 and 14D.
Thus, the heat can be transported in the opposite direction of the traveling acoustic wave by a round of the cycle of D.fwdarw.A.fwdarw.B.fwdarw.C.fwdarw.D shown in FIG. 13. The cycle constituted by an isothermal change and an isovolumetric change is called the Stirling cycle, corresponding to the Carnot cycle having the adiabatic change replaced with the isovolumetric stroke.
Therefore, efficiency of a level equal to that of the Carnot cycle can be obtained in the above acoustic refrigeration apparatus.
However, this acoustic refrigeration apparatus has a portion of the pressure wave reflected without passing through the regenerator. Therefore, there was a problem that the efficiency of the apparatus is degraded by the energy of the reflected pressure wave. Furthermore, when a plurality of the above acoustic wave generation devices are provided, a device to adjust the phase of these acoustic wave generation devices is required.